1. Field of the Invention
The present invention relates generally to batteries and, more particularly, to determining the remaining capacity of a battery.
2. Related Art
Sudden cardiac arrest has been attributed to over 350,000 deaths each year in the United States, making it one of the country's leading medical emergencies. Worldwide, sudden cardiac arrest has been attributed to a much larger number of deaths each year. One of the most common and life threatening consequences of a heart attack is the development of a cardiac arrhythmia, commonly referred to as ventricular fibrillation. When in ventricular fibrillation, the heart muscle is unable to pump a sufficient volume of blood to the body and brain. The lack of blood and oxygen to the brain may result in brain damage, paralysis or death to the victim.
The probability of surviving a heart attack or other serious heart arrhythmia depends on the speed with which effective medical treatment is provided. If prompt cardiopulmonary resuscitation is followed by defibrillation within approximately four minutes of the onset of symptoms, the probability of survival can approach or exceed fifty percent. Prompt defibrillation within the first critical minutes is, therefore, considered one of the most important components of emergency medical treatment for preventing death from sudden cardiac arrest.
Cardiac defibrillation is an electric shock that is applied to the victim to arrest the chaotic cardiac contractions that occur during ventricular fibrillation, and to restore a normal cardiac rhythm. To administer such an electrical shock to the heart, defibrillator pads are placed on the victim's chest, and an electrical impulse of the proper magnitude and shape is administered to the victim through the pads. While defibrillators have been known for years, they have typically been complicated, making them suitable for use by trained personnel only.
Recently, portable and transportable automatic and semi-automatic external defibrillators (generally, AEDs) for use by first responders have been developed. A portable defibrillator allows proper medical care to be given to a victim earlier than preceding defibrillators, increasing the likelihood of survival. Portable AEDs may be brought to or stored in an accessible location at a business, home, aircraft or the like, available for use by first responders. With recent advances in technology, even a minimally trained individual can operate conventional portable defibrillators to aid a victim in the critical first few minutes subsequent to the onset of sudden cardiac arrest.
Portable defibrillators require a portable energy source to operate in the anticipated mobile environments. Typically, AEDs utilize battery packs as the portable energy source. Several manufacturers have provided battery packs for use in portable defibrillators, such as a sealed lead acid (SLA) battery, a nickel cadmium (NiCd) battery, a lithium ion (Li) battery, and the like.
AEDs can remain unused in their storage area for hours, days or even weeks without having been tested or otherwise maintained, and months or even years without actually being used. During such extended periods of time, the installed battery may discharge significantly and have insufficient energy to charge the AED during use. Thus, to ensure reliable AED operations, it is important that the condition of the installed battery be determined during use. The user can then determine when to replace the battery.
A conventional technique for measuring remaining battery capacity is commonly referred to as a battery fuel gauge. With this technique, a battery monitoring circuit measures the current output from the battery during use, and, in the case of rechargeable batteries, the current input to the battery during charging cycles. The battery monitoring circuit determines the remaining capacity of the battery based on a tally of the cumulative input and output currents.
A drawback to this approach is that this approach requires the use of an expensive, low-power analog and digital circuit designed to monitor the battery continually to calculate self-discharge quantities during the storage of the device. Another significant drawback is that the requisite current sensor is lossy, reducing the efficiency of the device. Furthermore, due to risks commonly associated with such a technique, a supplemental measurement technique is often implemented concurrently. In addition, the test device must be re-calibrated periodically to adjust for changes in the battery capacity over the cycle life of a rechargeable battery.